She's not nervous about speaking to a lot of people, but speaking to a TV camera is a different kettle of fish.

她在许多人面前讲话都不紧张, 但在电视摄影机前面讲话就完全是两回事了.

《简明英汉词典》

However, the essays are a different kettle of fish.

然而, 论文是截然不同的事.

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The Antarctic, however, is a different kettle of fish.

但在南极地区捕鲸却是另外一回事.

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That's a different kettle of fish.

那是另一回事.

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The new proposal is quite a different kettle of fish from the last one.

新建议与旧建议完全是两回事.

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